How does a tube's need for bias change with age?
I'm not exactly sure how to phrase this with the proper terminology, but I was wondering if a tube's need for bias changes in a consistent direction with the age of the tube. In other words is there a common trend universal to all tubes? For example if you check bias after one year of use is there a trend to need to correct bias by increasing negative bias voltage, or decrease it? Or does it vary depending on tube type? If you were to set bias on the hot side to begin with would it tend to cool over time? Again apologies if the terminology is wrong or if this is a moronically basic question.
In general the valve will get weaker so will need a decreased negative bias to maintain a given anode current. Of course something might go wrong with it and the opposite happens, but that's a fault. Also new and NOS valves may need frequent re-biasing in either direction as they settle in.
Cathode emission drops with age, so for a given bias, the current will drop.
The peak current the cathode will handle also drops.
Many of the last generation of valves had outsize cathodes relative to what they actually needed, which explains how a 25CT3, a tiny noval diode can pass 230m/a, has half the voltage drop c/w the whimpy 6X4, and outlasts it by a large margin.
Short answer is it depends on the tube.
Back in the day major manufacturers (RCA, Stlvania, GE, ect) had good QC and burn in procedures, so the tube was already correctly aged right out of the box and it would be quite stable. Now you have mass produced tubes made eastern bloc countries and China, with next to no QC. I've had brand new tubes redplate and bias incorrectly on a regular basis, so I watch them like a hawk until they make me sleep easy at night.
If you care about your equipment as I do you'll want to do your own verification, because in effect you are the QC department. You're the final say as to whether a given tube will perform correctly, so I always do the following before I put an amplifier or vintage TV into regular rotation:
Perform emission test on a tube tester
Put pairs/quads into an amp I don't care about first, so an arc doesn't take out an irreplaceable output transformer
Check actual bias against plate curves in data sheets, to see if they actually are working the way they're supposed to
Put them into the gear they are destined for, power up and set bias per gear's manual
Observe tube plates in a totally dark room, to make sure they aren't replating at idle (you'd be surprised how many fail this part)
If plates are red, reject and demand a replacement (only exceptions are transmitting tubes, and only if the data sheets say glowing plates are permissible)
If all of that goes well, I put audio through it and give it a torture test at high output levels all the while looking for problems. I'm very demanding of my equipment, I've paid good money for it so I expect a certain level of performance and longevity out of it. If you don't, plug them in and cross your fingers...
And yes, emission will drop with age. I usually redo bias each year, YMMV. My stuff doesn't get much use, but when it does I push it as hard as it was designed to be pushed. It's like car maintenance, take care of it regularly and you'll get good mileage.
I purchase tubes that have been reasonably pre-aged by the manufacturer, and then after crossing the ocean, the local vendor ages them a little more, and then they are retested.
My experience with those tubes has been that the tube current does not significantly change for many many hundreds of hours over a 1 or 2 year period (including the warming and cooling effects of turning them on and off many many times).
I have used both fixed bias, and self bias on these tubes. The current has been stable.
So what tubes are you using?
What are the operating conditions (quiescent current, voltage, plate and screen dissipation; and what kind of signal levels versus max power are they putting out)?
The more you abuse a tube, the quicker it settles in to a non-working condition.
Look at the lifetime of old Hi Fi amplifier tubes that operate for decades, versus a Heavy Metal Rock Guitar amp that exceeds the tube ratings of voltage, current, screen current, and plate current.
Not to mention the hard clipping nature of a run-away concert. And these tubes run Hot,
generally without proper air flow, and then have to cool from that overheated condition.
Cathode Coating fatigue?
I just don't agree at all with the above...."back in the day" just doesn't cut it.
My experience with NEW, is there is tremendous spread in production....
No aging or "burn in" has ever taken place...
You have a lot of valves in the middle of the curve, some qty weaker, a good quantity stronger, and at the other end, valves which are impossibly strong, so they get hot. Sometimes they will calm down, but if after 15mins it's still not dropping, they go on one side for other duties.
I have seen this with Sylvania 8417, they have NEVER been run, and in some cases run excessively strong...had it also on NEW 70yr old KT8C...never been run ever.....and when in a typical amp runs so strong I can't match it to anything else.
When you start up one of these new stock valves you see a burst of emission to begin with, then they slowly settle.
I reckon at worst it takes 5 mins,- after this point it's just not going to move any more.
The whole ****-eyed story that "burning in takes weeks" and the sound alters is entirely imaginary...(as I can clearly see on a meter) but there, you have to humour people who believe in fairy stories...
Fairy stories are sometimes closer to home than we realise. I have many, many old valves and have tried to keep as many as possible usefully working. My experience is that valves with little use for several decades are often erratic at first, especially those still boxed. The issue is for how long I persevere. I have tried burning in (or out) valves on full emmissions for over 24 hours in hope, yet many remain stubbornly variable and as a technique it is flawed. Maybe outgassing, maybe substandard stock which got shelved, who knows why. Some did come right, some got binned. I'd say 50 hours is sufficient for any valve worth persevering with in order for it to settle down. I wouldn't give a new valve that leeway, it should perform to spec out of the box. But old stock is different. Not one size fits all.
I have another 200 more of the finest to select out over new year.
These were made in 1973, - the best possible imagineable quality GEC-Marconi.
I don't see anything like the phenomenon you describe.
I can assure you the turnaround time of the test sequence (and NO it's not a valve tester it's an amp running at full voltage and proper bias)...is relatively long per valve, because they don't plug in...(no bases).
If I had the slightest doubt the bias would wander after 5mins I would entirely have to change the test sequence, which makes my job at least 10-12hrs longer for that quantity.
So, - as I say, what I see is repeatedly the following...A burst of high emission (say about 10 secs) then it slowly settles, let's give it 5-10mins MAX..most drop into to about 14m/a
To give you some idea of spread, voltage drop over an anode resistor of 49ohms (detection) varies to give idle currents on my test varying from 11m/a (weak) to 19m/a (too strong) a variation of 70%.
Being as I am fitting them as matched quads, you can begin to imagine I don't have huge patience with the argument that what I might be doing would be pointless or stupid.
Once I have found a pair or a quad of them with identical parameters I don't expect this to change, and on tests up to full power over a week, nothing moved at all from the first 5 minute settle down period.....
Fact is running them at proper voltages and loads, 45yrs later they behave much as you would expect.
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